Horizontal liquid phase crystal growth apparatus

ABSTRACT

A horizontal liquid phase crystal growth apparatus which incorporates a heat sink means is described. The apparatus includes a boat for housing a melt which is in contact with a slider used for accommodating a source material. The slider, in turn, is in contact with a pedestal used for holding a substrate. The melt and the substrate are isolated from one another at different stages of the growth process by means of the slider. In operation, the boat is heated to a growth temperature and the slider is then moved so as to effect saturation of the melt with the source material. The slider is then moved again to contact the substrate with the source-saturated melt. Growth is then initiated from a convection-free melt by cooling the pedestal and, thus, the substrate by means of a heat sink which establishes a thermal gradient along the vertical axis of the container.

United States atent Bergh et al.

[54] HORIZONTAL LIQUID PHASE CRYSTAL GROWTH APPARATUS [72] Inventors: Arpad Albert Bergh, Murray Hill; Carl Ralph Paola, Westfield, both of NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

[22] Filed: Mar. 16, 1970 [21] Appl. No.: 19,878

52 u.s.c| ..1l8/58,23/273SP,118/415, l48/l7l 51 lnt.Cl. ..B05c3/02 [58] Field of Search ..23/273 SP, 301 SP; 118/422, 118/423, 425, 58, 64, 412; 148/415,171

[451 May 30, 1972 3,507,625 4/1970 Deyris ..23/301 3,551,219 12/1970 Panishetal ..ll8/422 Primary Examiner-Norman Yudkofi' Assistant Examiner-R. T. Foster Attorney-R. J. Guenther and Edwin B. Cave [57] ABSTRACT A horizontal liquid phase crystal growth apparatus which incorporates a heat sink means is described. The apparatus includes a boat for housing a melt which is in contact with a slider used for accommodating a source material. The slider, in turn, is in contact with a pedestal used for holding a substrate. The melt and the substrate are isolated from one another at different stages of the growth process by means of the slider. In operation, the boat is heated to a growth temperature and the slider is then moved so as to effect saturation of the melt with the source material. The slider is then moved again to contact the substrate with the source-saturated melt. Growth is then initiated from a convection-free melt by cooling the pedestal and, thus, the substrate by means of a heat sink which establishes a thermal gradient along the vertical axis of the container.

4 Claims, 8 Drawing Figures Patented May 30, 1972 5 Sheets-Sheet 1 Patented May 30, 1972 3,665,888

3 Sheets-Sheet 2 HORIZONTAL LIQUID PHASE CRYSTAL GROWTH APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a horizontal liquid phase crystal growth apparatus and more particularly to a horizontal ap paratus which incorporates a heat sink means to establish a vertical temperature gradient during growth.

2. Description of the Prior Art Heretofore, crystal growth from the liquid phase has been accomplished with a multiplicity of horizontal liquid phase apparatus. However, the quality of the regrown layers, especially in epitaxial growth has been adversely affected by the quality of the substrate material as well as by temperature gradients in the melt. Surface degradation of the substrate most often occurs from (1) decomposition of the substrate followed by volatilization of an element therefrom at elevated temperatures, (2) the transfer of volatile compounds from the melt to the substrate, and (3) the transfer of foreign particles to the substrate surface from the melt.

.An apparatus which restricts the free volume surrounding the substrate and which physically separates the substrate from the melt would alleviate the first two above-mentioned causes of surface degradation. Since most impurities have a lower density than the usual melts employed in liquid phase growth, small particles of contamination usually float atop the melt. Apparatus has been developed heretofore in which the melt is skimmed" prior to contacting the substrate; however, the clean surface required entails extreme care and it would be much easier and safer to construct an apparatus which prevents the substrate from contacting the liquid-gas interface thereby eliminating the third above-mentioned cause of surface degradation.

In addition to the above-noted difiiculties, convective currents in the melt during regrowth result in (I) an irregular thickness of the regrown surface, (2) a variable doping profile over the regrown layer, and (3) a damaged interface between the substrate and the regrown layer. If cooling starts at the top of the melt or if regrowth is initiated at the top, gravity causes the heavier solution to sink and form convective cells. To avoid convection, a vertical temperature gradient must be established with a decreasing temperature toward the lowest part of the system. However, the' apparatus should have a horizontal temperature gradient of zero in order to obtain a uniform thickness of the regrown layer.

SUMMARY OF THE INVENTION The present invention is directed to a horizontal liquid phase crystal growth apparatus. The apparatus is one which optimizes the liquid phase crystal growth by (l) maintaining the free volume surrounding the substrate at a minimum, (2) physically segregating the substrate from the melt prior to growth, 3) preventing melt-substrate contact at the liquid-gas interface and (4) providing a vertical thermal gradient whereby the substrate is maintained at the lowest temperature.

The horizontal apparatus consists of a boat for containing a melt, said boat having a lid to prevent the evaporation of volatile components. In intimate contact with the boat is a slider which has a well for accommodating a source mixture. The slider combines the source mixture with the melt to form a source-saturated melt when the boat is maintained at a growth temperature. The slider has a conduit passageway or opening for channeling the source-saturated melt into contact with a substrate when the substrate and the melt are maintained at the growth temperature. The slider is in slideable contact with a base or pedestal having a first compartment for accommodating the substrate and a second compartment for accommodating any excess of thesaturated melt when the growth process is terminated by means of the slider. Contacting the pedestal is a heat-sink means which insures deposition from a convection-free melt.

In operation, a suitable substrate is placed into the first compartment of the pedestal. The slider is placed upon the pedestal whereupon the boat is placed upon the slider. The source material is then placed in the slider well and the melt mixture isplaced in the boat and covered with the lid. The boat,'with the lid in place, the slider and pedestal are then placed in a furnace and the boat is heated to pre-saturate and dope the resultant melt. The slider is then moved so that the melt comes in contact with the source material to form a source saturated melt. The temperature of the melt and the substrate, contained in the boat and the pedestal, respectively, are maintained at the growth temperature and the slider is moved again so that the conduit opening is in alignment with both the source-saturated melt and the substrate thereby permitting contact therebetween. Crystal growth is then initiated from the saturated melt by cooling the substrate, employing a heat-sink means. A vertical temperature gradient is established via the heat sink to insure deposition from a convection-free melt. The growth process can be terminated in a controlled fashion at any time by moving the slider so as to separate the source-saturated melt from the substrate.

DESCRIPTION OF THE DRAWING The present invention will be more readily understood by reference to the following drawing taken in conjunction with the detailed description, wherein:

FIG. 1 is a perspective view of the pedestal, slider, and the boatof the invention;

FIG. 2 is a perspective view of the slider slid into place with respect to the pedestal;

FIG. 3 is a perspective view of the assembled pedestal, slider and boat within a horizontal furnace;

FIG. 4A is a cross-sectional view of the crystal growth ap- DETAILED DESCRIPTION The present invention has been described largely in terms of the epitaxial growth of p-type Galv on a substrate of p-type GaP. However, it will be understood that suchdescription is for purposes of exposition and not for purposes of limitation. It will be readily appreciated that the inventive concept described is equally applicable to non-epitaxial as well as epitaxial growth and to crystal growth of non-semiconductor materials as well as semiconductor materials. Also the inventive concept described is applicable to many combinations of substrate and melt whereby both homojunctions and heterojunctions are formed. Regarding the epitaxial growth of semiconductor materials, the materials may be selected from among groups Ill(a) V(a) compounds, groups (12) VI(a) compounds or group IV elements of the Periodic Table of the Elements as set forth in the Mendelyeev Periodic Table appearing on page B2 of the 45th edition of the Handbook of Chemistry and Physics, published by the Chemical Rubber Company.

With reference now to FIG. 1, there is shown a pedestal 41, which can be fabricated from any inert material including such materials as high purity graphite, alumina, quartz, boron nitride, or any inert ceramic material. It is to be understood that all of the above-mentioned materials, including graphite, may or may not be employed with a high purity graphite liner (not shown). The base 42 of the pedestal 41 has a first compartment 43 destined for accommodating a suitable substrate 44 (FIG. 2), flushly with its walls 46. The base 42 has a second compartment or reservoir 47 destined for accommodating a portion of a suitable crystal growth melt 45 (FIG. 415) when the crystal growth is terminated.

The sides 48-48 and base 42 of the pedestal 41 define a slide surface 49 which is the top surface of the base 42. The slide surface 49 is destined for accommodating a slider 51 which is made of the above-mentioned inert materials. The slider 51 comprises a base 52 having a member 53 perpendicular thereto on one end thereof. The base 52 has a well 54 destined for accommodating a source mixture 56 (FIG. 2). Extending through the base 52 is a channeling conduit or aperture 57 which is destined for channeling the melt 45 (FIG. 4D) employed in the crystal growth to the substrate 44.

Sides 48-48 of the pedestal 41 have grooved slideways 58 which are destined for accommodating a boat 59. The boat 59, which is made of the above-mentioned inert materials, has a first compartment 61 destined for holding a melt mixture 62 (FIG. 3) and a channeling compartment 63 destined to be loaded with the source mixture 56 for channeling to well 54 (FIG. 4A). Separating compartment 61 from compartment 63 is a spacer compartment 60. The bottom 68 of compartment 60 is destined to serve as a cover for well 54 during stages of the crystal growth procedure (FIG. 4B). The boat 59 has two ridges 64-64 on opposed sides, which are designed to fit into and slide along the grooved slideways 58-58. Covering compartment 61 is a lid 65 which is constructed of the above-mentioned inert materials and which serves to prevent the evaporation of volatile elements from melt 62 (FIG. 4B) and melt 45 (FIG.4C).

Referring to FIG. 2, prior to crystal growth, the slider 51 is placed at one end 67 of the pedestal 41 and slid along slide surface 49 to a predetermined point. The slider 51 is destined to pass over and cover compartment 43 containing substrate 44 (FIG. 4B). In this regard, it is to be noted that when the substrate 44 has been inserted into compartment 43, the top surface of the substrate 44 is almost flush with the slide surface 49 of the base 42. Therefore, when the slider 51 covers the substrate 44, the free volume surrounding the substrate 44 is thereby maintained at a minimum. This minimization of the free volume decreases the elemental loss due to evaporation from the substrate 44. The free volume should be such as to ideally restrict the loss to one monolayer of the surface of the substrate 44. Therefore, the free volume allowed is dependent upon the substrate material chosen and the partial vapor pressures involved.

' Referring to FIG. 3, after slider 51 has been slid into position in relation to pedestal 41, the boat 59 is placed at the end 67 of the pedestal 41 and the ridges 64 are inserted into slideways 58. The boat 59 which is in intimate contact with the top surface 69 of the base 52 of the slider 51 is then slid along slideways 58 until the front end of the boat 59 abuts member 53 which prevents the boat 59 from sliding further. It is to be understood that the top surface 69 is in such intimate contact with the boat 59 that the compartment 61 may now hold a melt mixture 62 without likelihood of loss. When the boat 59 is slid into place, two stop bars 50 and 55 are inserted into slots 71 and 72, respectively (FIGS. 1 and 3).

As indicated in FIG. 3, the assembled pedestal 41, slider 51 and boat 59 are destined for placement into a standard horizontal furnace 74. Incorporated in the furnace 74 is a heat exchanger 77. The heat exchanger 77 is in contact with the base 42 contiguous to compartment 43 containing the sub- .strate 44. Gas maintained at a low temperature is introduced via inlet and outlet means (not shown) through the heat exchanger 77. It is to be understood that although the heat exchanger 77 has been described in terms of a cooling gas exchanger, the inventive embodiment is not to be so restricted and any heat sink means may be employed.

Referring now to an exemplary technique, a suitable p-type doped GaP substrate material grown by standard liquid encapsulated pulling techniques was cut to size. It was lapped and cleaned in accordance with conventional techniques. Referring to FIG. 1, the lapped and cleaned GaP substrate 44 was inserted into a compartment 43 of a high purity graphite pedestal 41.

Referring to FIG. 2, a slider 51 fabricated of high purity graphite was slid to a predetermined point along the slide surface 49, into contact with the pedestal 41. A high purity graphite boat 59 was then slid onto the slider 51 by inserting ridges 6464 into slideway 58-58, as shown in FIG. 3. The boat 59 was slid along slider 51 until channeling compartment 63 was in alignment with well 54. In regard to the pedestal 41, the slider 51 and the boat 49, it was found that the optimal wall thickness for graphite construction was a maximum of oneeighth of an inch.

A source mixture 56 of Ga,0, and GaP was prepared by first weighing out 0.2 grams of high purity Ga,O obtained from commercial sources. The high resistivity GaP material was cut and 0.2 grams was added to the previously weighted out Ga O thus forming the source mixture 56. The source mixture 56 was then placed in compartment 63 and thus channeled into well 54.

A galliumGaP-Ga,O -Zn melt 'mixture 62 was prepared by first weighing out 8 grams of high purity gallium, 0.2 grams zinc, and 0.1 grams 611,0 obtained from commercial sources. To this was added 0.1 grams GaP. The resultant mixture was placed in compartment 61 of boat 59. The combined amount of Ga? present in the melt mixture 62 and the source mixture 56 was such as to give a GaP saturated gallium solution doped with oxygen and zinc at the growth temperature of 980 C. I

Upon loading the boat 59, a high purity graphite lid 65 was placed over compartment 61 and into contact with the melt mixture 62, and stop bars 50 and 55 were placed into slots 71 and 72, respectively. It is to be noted that the loading and assembly of the pedestal 41, slider 51 and boat 59 was done in a chamber (not shown) in which a dry and inert ambient was maintained. The loaded assembly was thereupon placed into a standard horizontal furnace 74, as shown in FIGS. 3 and 4A.

Proper adjustment of the furnace 74 assured an isothermal control position which accommodated the loaded assembly. An ambient atmosphere of nitrogen was passed into and out of the furnace 74 by means not shown. Thereafter, the nitrogen was cut off and hydrogen was passed into the system.

Referring to FIG. 4B, the slider 51 and thereupon boat 59 were moved by pushing one end of the base 52 of the slider 51 so as to align compartment 61 of boat'59 with compartment 43 of the pedestal 41, thereby aligning the substrate 44 with melt mixture 62. It is to be noted that boat 59 was now abutting stop bar 50 and was thus prevented from any further movement forward. The furnace 74 was heated to the growth temperature of 980 C thereby forming a melt 62. At this stage of the procedure, the bottom 68 of the spacer compartment 60 acts as a lid which covers well 54 and the source mixture 56. The free volume surrounding the source mixture 56 was thus kept at a minimum thereby preventing volatilization loss from the source mixture 56. In this regard, the slider 51 covers the substrate 44 and maintains the free volume surrounding the substrate 44 to a minimum.

Referring to FIG. 4C, following the formation of melt 62, slider 51' was moved forward. Since the boat 59 is abutting stop bar 50, the bar 50 prevents the boat 59 from moving forward with the slider 51. The slider 51 was moved so as to align compartment 61 with well 54 and thus combine the melt 62 contained in compartment 61 (FIG. 48), with the source 56,

. contained in well 54 (FIG. 413), to form a source-saturated melt 45.

The slider 51 was maintained in its position until thermal equilibrium was established between the substrate 44 and the source-saturated melt 45. Referring to FIG. 4D the slider 51 was moved forward again. The boat 59 was prevented from moving by the stop bar 50 and when compartment 61 was aligned with the conduit or aperture 57 of the slider 51, the source saturated melt 45 was channeled into contact with the substrate 44. The substrate 44 and the source-saturated melt 45 were at the same temperature and, therefore, neither deposition on the substrate 44 nor dissolution of the substrate occurred. The substrate was also prevented from surface contamination via contact with the surface of the melt.

A cooled inert gas, such as N was flowed through the heat exchanger 77 by means not shown. This caused the temperature of the base 42 contiguous to compartment 43 and substrate 44 to be lowered. This cooling established a vertical temperature gradient along the vertical axis of the pedestal 41 contiguous to compartment 43, with both the base 42 and the substrate 44 having the lowest temperature. Crystal growth was thus initiated by cooling the source-saturated melt 45.

The growth process was terminated in a controlled manner, as illustrated in FIG. 4B, by pulling the slider 51 back and thus separating the source-saturated melt 45 from the substrate 44. The layer of source-saturated melt 45 trapped in the slider 51 is deposited into the reservoir 47. The GaP substrate 44 was kept in contact with the melt 45 from to 60 minutes leading to a grown crystal layer of from one-fourth to 3 mils. In order to remove the apparatus from the furnace 74 (FIG. 3), the slider 51 was pulled back, so that member 53 contacted boat 59 which in turn abutted stop bar 55 thereby enabling the assembled pedestal 41, slider 51 and boat 59 to be extracted.

What is claimed is:

1. Horizontal liquid phase crystal growth apparatus including (a) a pedestal having a first chamber for accommodating a substrate member and a second chamber for accommodating a portion of a crystal growth melt, (b) a slide member comprising a base having a well for accommodating a source solution and a channeling conduit for channeling a melt to the first chamber of said pedestal, (c) a boat having a covered first compartment for containing a melt mixture, a second compartment for channeling a source solution to the well of said slider and a spacer compartment intermediate said first and second compartments, and (d) means for heating said boat, said pedestal being in intimate contact with said sliding member which in turn is in intimate contact with said boat, the pedestal also being in contact with heat sink means for establishing a vertical thermal gradient along the vertical axis of said pedestal.

2. Apparatus in accordance with claim 1 wherein the first chamber of said pedestal is adapted to intimately accommodate the sides of said substrate.

3. Apparatus in accordance with claim 1 wherein said heating means is a horizontal furnace.

4. Apparatus in accordance with claim 3 wherein said heat sink means is a heat exchanger maintained within said furnace. 

1. Horizontal liquid phase crystal growth apparatus including (a) a pedestal having a first chamber for accommodating a substrate member and a second chamber for accommodating a portion of a crystal growth melt, (b) a slide member comprising a base having a well for accommodating a source solution and a channeling conduit for channeling a melt to the first chamber of said pedestal, (c) a boat having a covered first compartment for containing a melt mixture, a second compartment for channeling a source solution to the well of said slider and a spacer compartment intermediate said first and second compartments, and (d) means for heating said boat, said pedestal being in intimate contact with said sliding member which in turn is in intimate contact with said boat, the pedestal also being in contact with heat sink means for establishing a vertical thermal gradient along the vertical axis of said pedestal.
 2. Apparatus in accordance with claim 1 wherein the first chamber of said pedestal is adapted to intimately accommodate the sides of said substrate.
 3. Apparatus in accordance with claim 1 wherein said heating means is a horizontal furnace.
 4. Apparatus in accordance with claim 3 wherein said heat sink means is a heat exchanger maintained within said furnace. 